A hydraulic control valve for a construction machine in accordance with the prior art as shown in FIGS. 3 to 5 includes:
a valve body 1 that includes a port Pi for a pilot pump, through which a pilot pressure is introduced, a tank port T through which the pilot pressure is drained, and a secondary pilot pressure port C configured to selectively fluidically communicate with the port Pi for the pilot pump or the tank port T;
a shifting device 3 (e.g., traveling pedal) that is rotatably mounted on the valve body 1 by means of a fixing pin 2;
a rod 4 that is configured to be shifted in cooperation with the shifting device 3 when the shifting device 3 is pressedly rotated ;
a spool 5 that is configured to be shifted in response to the shifting of the rod 4 to cause the port Pi for the pilot pump and the secondary pilot pressure port C to fluidically communicate with each other to set a secondary pilot pressure in proportion to a shifting amount of the shifting device 3; and
a valve spring 6 that is configured to elastically support the spool 5 to cause the secondary pilot pressure port C and the tank port T to fluidically communicate with each other.
A hydraulic circuit used for embodying a hydraulic control valve in accordance with the prior art as shown in FIGS. 1 and 2 includes:
a main hydraulic pump 7 (hereinafter, referred to as “hydraulic pump”) and a pilot pump 8, which are connected to an engine (not shown);
a hydraulic actuator 9 (e.g., traveling motor) that is connected to the hydraulic pump 7;
a main control valve (MCV) 10 that is installed in a path between the hydraulic pump 7 and the hydraulic actuator 9 and is configured to be shifted to control a start, a stop, and a direction change of the hydraulic actuator 9;
a control valve 12 that is installed in a signal pressure path 11 connected to a path 8a of the pilot pump 8 and is configured to be shifted to shift a spool of the main control valve 10 in a direction where the control valve 12 is shifted;
a pilot control valve 13 that is installed in the pilot pump 8 and the control valve 12 and is configured to set a secondary pilot pressure in proportion to the pressed rotation of the shifting device 3 during the shifting of the spool 5; and
a relief valve 14 that is installed in the path 8a of the pilot pump 8 and is configured to set a discharge pressure of the pilot pump 8.
When the shifting device 3 is pressedly rotated about a fixing pin 2 in a counter-clockwise direction on the drawing sheet of FIG. 7 to drive a hydraulic actuator 9, the rod 4 is shifted to the bottom on the drawing sheet in cooperation with the shifting device to cause the spool 5 to be shifted to the bottom on the drawing sheet. At this point, the valve spring 6 receives a compressive force. For this reason, a hydraulic fluid of the port Pi side for the pilot pump 8 is transferred to the secondary pilot pressure port C after sequentially passing through an orifice 15, a first path 16, and a second path 17 in this order, so that a secondary pilot pressure is formed in a signal pressure path 11. In other words, the secondary pilot pressure formed in the signal pressure path 11 rises in proportion to a shifting amount of the shifting device 3 when the shifting device 3 is pressedly rotated downwardly.
As mentioned above, in case of the hydraulic control valve that forms the secondary pilot pressure in the signal pressure path 11 in proportion to the shifting amount of the shifting device 3, a high pressure is generated in the main control valve 10 that controls a hydraulic fluid supplied to the hydraulic actuator 9 from the hydraulic pump 7. In this case, when leakage of the hydraulic fluid occurs through a gap defined between a valve body and a spool of the main control valve 10, a hydraulic fluid from an external connection device (e.g., a hydraulic port of the main control valve) connected to the secondary pilot pressure port C can back-flow to the secondary pilot pressure port C via shifting signal paths a and b, and the signal pressure path 11. In this case, there is caused a problem in that the pilot hydraulic parts (e.g., the signal pressure path 11, a spool cap of the main control valve 10, and the like) to which the pilot pressure is supplied are damaged or a failure thereof is induced due to the high pressure formed in the secondary pilot pressure port C.